The present invention relates to an acceleration sensor for use with an automotive air bag system or similar occupant protection system and, more particularly, to a mechanical acceleration sensor for sensing an acceleration or a deceleration by using a weight which is movable due to inertia in the event of acceleration or deceleration.
Many of modern motor vehicles are equipped with various implementations for the protection of occupants such as an air bag system. In general, an air bag system causes a bag to inflate in the event of a collision by sensing an impact force applied to a vehicle body, i.e., a deceleration of a vehicle body greater than a predetermined degree. With an air bag system, therefore, it is necessary to use an acceleration sensor which is sensitive to decelerations greater than the a predetermined value.
A predominant type of acceleration sensor available with an air bag system uses a weight which is movable by inertia, as disclosed in Japanese Patent Publication No. 52-13104 (corresponding U.S. Patent Ser. No. 32,090 and British Patent No. 1,312,609) by way of example. In such a mechanical acceleration sensor, the weight is constantly biased by a spring so that it may be usually held in a predetermined position. The weight in turn retains a firing pin or similar actuating member in an inoperative or initial position. When the vehicle body undergoes a deceleration greater than a predetermined value which is representative of a collision, the weight is shifted by a predetermined amount against the action of the spring so as to release the actuating member. Then, the actuating member is driven by a compression spring to impact against a detonator which initiates the expansion of the inflatable bag. It has been customary to cause the weight and actuating member to move linearly along a guide.
A prerequisite with an air bag system is that the system be prevented from being actuated by low-speed collisions, ordinary rapid decelerations, vibrations of a vehicle body which are not derived from a collision, etc. Upon actual collision, however, the system has to be surely triggered. These requirements cannot be satisfied unless the acceleration sensor is capable of operating with accuracy, i.e., unless the displacement of the weight is accurately designed for a predetermined deceleration. Also, the actuating member needs to actuate the system immediately as soon as it is freed from the restraint.
A problem with the prior art acceleration sensor having a weight and an actuating member which are linearly movable is that the weight and actuating member and the guide undergo friction due to their sliding contact. The friction is substantial and, moreover, it varies with the moving speed of the weight and/or that of the actuating member. It is therefore extremely difficult to adequately select the preload of the spring for controlling the movement of the weight and the preload of the spring for driving the actuating member. Another problem with such a prior art acceleration sensor is that the structure is complicated because independent springs are needed for controlling the weight and for driving the actuating member.